Two-Step Spin Transition in a 1D FeII 1,2,4-Triazole Chain Compound

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(beta Alatrz)(3)](BF4)(2)center dot 2H(2)O (1 center dot 2H(2)O), whose precursor beta Alatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a beta-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), Fe-57 Mossbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt twostep spin crossover (T-1/2(down arrow) = 230K and T-1/2(up arrow) = 235 K, and T-1/2(down arrow) = 172 K and T-1/2(up arrow) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe-II 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1 center dot 2H(2)O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1 center dot 2H(2)O. Vibrational spectra of 1 center dot 2H(2)O reveal that the BF4- anions and water molecules play no significant role on the vibrational properties of the [Fe(beta Alatrz)(3)](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(beta Alatrz)(3)](BF4)(2) (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.